1. Field of the Invention
The present invention relates to luminescent synthetic polymers. More particularly, the invention relates to photoluminescent, thermoluminescent and electroluminescent polymer blends useful for screen printing.
2. Description of Related Art
Silk screen printing has its origins in Japanese stenciling, but the screen printing process that we know today probably stems from the patents taken out by Samuel Simon of Manchester approximately 100 years ago. He used silk stretched on frames to support hand painted stencils, a process also used by William Morris. In 1914 John Pilsworth of San Francisco also took out a patent for multicolor printing, using the screen process.
The term “luminescenz” was first used in 1888 by Eilhardt Wiedemann, German physicist and historian of science, for “all those phenomena of light which are not solely conditioned by the rise in temperature.” By the rise in temperature, Wiedemann referred to the fact that liquids and solids emit more and more radiation of shorter and shorter wavelengths as their temperature increases, finally becoming perceptible to the eye as the material becomes red hot and then white hot. This is incandescence or “hot light,” in contrast to luminescence or “cold light.”
Examples of luminescence are the dim glow of phosphorus (a chemiluminescence), the phosphorescence of certain solids (phosphors) after exposure to sunlight, X-rays or electron beams, the transitory fluorescence of many substances when excited by exposure to various kinds of radiation, the aurora borealis and the electroluminescence of gases when carrying a current, the triboluminescence of crystals when rubbed or broken, the bioluminescence of many organisms, including the firefly, the glowworm and the “burning of the sea,” the fungus light of decaying tree trunks, and the bacterial light of dead flesh or fish.
For centuries incandescence was the universal method of artificial illumination: the torch, candle, oil lamp, gas lamp and tungsten filament served to light the way. There remains a need for a useful, renewable cold light source, particularly for photoluminescent materials which will absorb light and then emit useful amounts of light over long periods, thermoluminescent materials in which the photoluminescence is activated by heat and electroluminescent materials in which the light output is in response to electrical current.
Phosphorescent pigments are those in which excitation by a particular wavelength of visible or ultraviolet radiation results in the emission of light lasting beyond the excitation. After cessation of luminescence and renewed exposure to light, the material again absorbs light energy and exhibits the glow-in-the-dark property (an absorbing-accumulating-emitting cycle). Most phosphorescent pigments suffer from the problems of low luminescence and/or short afterglow.
Various phosphorescent substances are known, including sulfides, metal aluminate oxides, silicates and various rare earth compounds (particularly rare earth oxides). The most common type of phosphorescent pigment is zinc sulfide structure with substitution of the zinc and activation by various elemental activators. It is known that many luminescent materials may be prepared by incorporating metallic zinc sulfide (which emits green light). Moreover, with zinc sulfide a material or mixtures of materials variously termed activators, coactivators or compensators are usually employed. Known activators include such elements as copper (forming ZnS:Cu, probably the most common zinc sulfide phosphor), aluminum, silver, gold, manganese, gallium, indium, scandium, lead, cerium, terbium, europium, gadolinium, samarium, praseodymium or other rare earth elements and halogens. These activators presumably enter the crystal lattice of the host material and are responsible for imparting the luminescent properties to the material. Other sulfide phosphors which emit various colors of light include ZnCdS:Cu and ZnCdS:Ag, CaS:Bi, CaSrS:Bi, alpha barium-zinc sulfides, barium-zinc-cadmium sulfides, strontium sulfides, etc. The other important class of long-life phosphorescent pigments is the metal aluminates, particularly the alkaline earth aluminate oxides, of formula MAl2O4 where M is a metal or mixture of metals. Examples are strontium aluminum oxide (SrAl2O4), calcium aluminum oxide (CaAl2O4), barium aluminum oxide (BaAl2O4) and mixtures. These aluminate phosphors, with or without added magnesium, may be further activated with other metals and rare earths.
For example, U.S. Pat. No. 5,558,817 (1996) to Bredol et al. discloses a method of manufacturing luminescent zinc sulfide of cubic structure activated by copper and aluminum, forming a material having a high x-value of the color point as well as a high luminous efficacy in conjunction with a simple manufacture. U.S. Pat. No. 3,595,804 (1971) to Martin, Jr. discloses a method for preparing zinc sulfide and zinc-cadmium sulfide phosphors containing aluminum and activated with silver or copper. U.S. Pat. No. 3,957,678 (1976) to Dikhoff et al. discloses a method of manufacturing a luminescent sulfide of zinc and/or cadmium. The luminescent sulfide may be self-activated or activated by silver, copper and/or gold and coactivated by aluminum, gallium, indium, scandium and/or the rare earths. U.S. Pat. No. 3,970,582 (1976) to Fan et al. discloses luminescent materials comprising alpha barium zinc sulfides or barium zinc cadmium sulfides activated with manganese, europium, cerium, lead or terbium and methods for making the phosphors.
Alkaline earth metal aluminate oxide phosphors and their preparation are discussed in U.S. Pat. No. 5,424,006 to Murayama et al. Alkaline earth aluminum oxide phosphors of formula MAl2O4 were prepared where M was selected from calcium, strontium, barium or mixtures thereof, with or without added magnesium. The phosphorescent aluminates were activated with europium and co-activated with lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, tin, bismuth or mixtures thereof. These metal aluminate phosphors have a bright and long-lasting photoluminescent afterglow and show a glow peak of thermoluminescence in a high-temperature region of 50° C. or above when irradiated by ultraviolet or visible rays having a wavelength of 200 to 450 nm at room temperatures.
The alkaline earth metal type aluminate phosphors of Murayama et al. were developed in response to the problems with zinc sulfide phosphors decomposing as the result of irradiation by ultraviolet (UV) radiation in the presence of moisture (thus making it difficult to use zinc sulfide phosphors in fields where it is placed outdoors and exposed to direct sunlight) and problems of insufficient length of afterglow (necessitating doping a radioactive substance to the phosphorescent phosphor and employing a self-luminous paint which keeps emitting light by absorbing radiation energy for items such as luminous clocks). The metal aluminate phosphors such as activated alkaline earth aluminate oxides exhibit UV insensitivity and bright and long-lasting afterglow luminance. However, metal aluminate phosphors may be at a disadvantage compared to zinc sulfide phosphors in requiring a considerably long time and/or more intense illumination for excitation to attain saturation of afterglow luminance and vulnerability to water and moisture. This points out is the need for adaptation of specific phosphors and mixtures of phosphors for use in varying excitation conditions, a need for water-resistant formulations suitable for protecting phosphorescent particles and a need for UV protection where sulfides are utilized.
Phosphorescent materials have found use in a variety of commercial applications including warning signs, machinery marking, dial illumination, directional signs, marking the edge of steps, fire helmets, accident prevention, protective clothing, sports equipment, etc. Commercially available sheets of phosphorescent material are typically phosphorescent pigment in clear polyvinylchloride. Other approaches are also utilized, usually involving thermoplastics (which may be repeatedly softened by heating and hardened by cooling) or elastomeric and rubbery materials.
For example, U.S. Pat. No. 4,211,813 (1980) to Gravisse et al. discloses photoluminescent textile and other flexible sheet materials coated with a thin film of photoluminescent synthetic resin. A textile material was coated with a synthetic resin containing a phosphorescent metal sulphide and a substance which absorbs energy of short wave-length and emits energy at wave-lengths which lie within the absorption spectrum of the phosphorescent constituent. Preferred resins were polyurethane resins, polyvinyl chloride resins, polyacrylates and/or acrylates, elastomeric silicones and combinations of these resins. The preferred phosphorescent sulphide was zinc sulphide, with calcium, cadmium and strontium sulphides also being utilized. U.S. Pat. No. 5,692,895 (1997) to Farzin-Nia et al. discloses luminescent orthodontic appliances. A preferred orthodontic bracket material comprises a plastic material, preferably polycarbonate, glass fiber reinforcement and luminescent pigment, preferably zinc sulfide doped with copper or zinc sulfide doped with copper and manganese. U.S. Pat. No. 5,605,734 (1997) to Yeh discloses a method of making carpet with phosphorescent directional signals and signs. Symbols were tufted into the carpet using polymeric filaments and fibers containing zinc sulfide copper activated pigments.
U.S. Pat. No. 5,698,301 (1997) to Yonetani discloses phosphorescent articles composed of sequential layers of a transparent resin layer containing no UV light absorber, a phosphorescent layer utilizing SrAl2O4 as the phosphorescent pigment and a reflective layer, with an optional adhesive layer backing on the reflective layer. The transparent resin layer may be materials such as polycarbonates, acrylic resins, polyvinyl chlorides and polyesters. The phosphorescent layer is effected by dispersing the phosphorescent pigment in a varnish prepared by dissolving one of the above resins (preferably an acrylic resin or a vinyl chloride-acrylic copolymer resin) in a solvent and printing onto the transparent or reflective layer. U.S. Pat. No. 5,674,437 (1997) to Geisel discloses methods of making luminescent fibrous material by combining a metal aluminate oxide pigment with a thermoplastic polymer, which is heated, mixed and extruded into fibers. The luminescent comprises a thermoplastic polymer such as polypropylene, polyamides, polyesters, polymethacrylics, polyacrylates, polycarbonates, polycyanoethylenes, polyacrylonitrides, polyvinyl chloride, polyethylene, polystyrene, polyurethane, acrylate resins, halogenated polymers or mixtures. The metal aluminate oxide pigments are selected from strontium, calcium or barium, with or without magnesium, and contain a europium activator and a co-activator of lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium tin or bismuth. A plasticizer is also added. U.S. Pat. No. 5,607,621 (1997) to Ishihara et al. discloses methods of making phosphorescent resins and formed articles. The phosphorescent comprises a resinous material such as acrylic resin, ABS resin, acetal homopolymer or copolymer resins, PET, polyamides such as nylon, vinyl chloride resin, polycarbonates, polyphenylene oxide, polyimide, polyethylene, polypropylene or polystyrene, an SrAl2O4 phosphorescent pigment and a liquid paraffin activator. The phosphorescent resin mixture was kneaded at a temperature higher than the melting point of the synthetic resin and extruded to produce pellets for injection or extrusion molding.
U.S. Pat. No. 5,716,723 (1998) to Van Cleef et al. discloses glow in the dark shoe soles of rubber (a styrenic block copolymer or butadiene block copolymers), processing oil (plasticizer or extender), stabilizer (ultraviolet stabilizers, antioxidants and/or preservatives) and phosphorescent material (preferably zinc sulfide copper compounds). Optional ingredients include flow modifiers, modifying polymers and fillers. U.S. Pat. No. 4,629,583 (1986) to Goguen also discloses phosphorescent polymer-containing compositions suitable for use in shoes. The composition includes an elastomeric polymer, a processing oil, a stabilizer and a phosphorescent pigment (preferably a zinc sulfide copper compound), with optional modifying polymers, dry blend flow modifiers and fillers. The elastomeric polymer is preferably a styrenic block copolymer, monoalkenylarene copolymer or polystyrene polybutadiene block copolymer. Preferred modifying polymers included high density polyethylene, ethylene vinylacetate, polybutadiene resins, high styrene resins, poly(alpha-methylstyrene) resin, crystal polystyrene resin, high impact styrene polymers and co-polymers and mixtures thereof.
Numerous other plastic articles containing phosphorescent materials are also known. For example, U.S. Pat. No. 3,936,970 (1976) to Hodges discloses light emitting fish lures. The luminescent material comprises a phosphor such as zinc sulfide, an extender such as magnesium carbonate for increased luminous life, a suspending agent such as silica and zinc palmitate and a carrier for the luminescent material such as a transparent or translucent plastic. U.S. Pat. No. 5,490,344 (1996) to Bussiere discloses glow-in-the-dark fishing lures made by combining a white powder with a plastic resin and a phosphorescent substance. Typical resins include thermoplastic rubber, styrenics, polyolefin and plastisol. U.S. Pat. No. 4,759,453 (1988) to Paetzold discloses a baby bottle marked with a luminescent marker band made of synthetic plastic to which has been added an inorganic zinc sulfide phosphor with double activators. U.S. Pat. No. 4,210,953 (1980) to Stone discloses a flashlight having a luminescent case, band or sleeve containing a zinc sulphide or zinc-cadmium sulphide phosphorescent material.
Polymer epoxies were utilized in U.S. Pat. No. 5,395,673 (1995) to Hunt, which discloses a composition useful for non-slip ground surfaces where lighting conditions may be poor. The composition preferably includes a polymer epoxy (diglycidyl ether resin aliphatic amine adduct modified with amyl ethyl piperidine as a stabilizer), a phosphorescent pigment (preferably copper activated zinc sulfide)and an aggregate such as aluminum oxide.
A much different approach, which points out the need for improved thermosetting luminescent resins, was taken in U.S. Pat. Nos. 5,135,591 (1992) and 5,223,330 (1993) to Vockel, Jr. et al. These patents disclose processes and phosphorescent fiberglass reinforced plastic articles in which a phosphorescent pigment is first applied to the reinforcing fabric using a carrier resin and then cured. Suitable carrier resins include acrylic latex, epoxy, polyvinylchloride, ethylenevinylchloride, polyurethane, polyvinylacetate, acrylonitrile rubber, melamine and co-polymers of these compounds. The phosphorescent coated fabric can then be utilized with both thermoplastic resins (which can be melted and reshaped with heat after curing) and thermosetting resins (which cannot be melted and reshaped with heat after curing) to make FRP (fiberglass reinforced plastic) products. This approach utilizing a phosphorescent fabric was taken for two reasons: 1) previous attempts to add a phosphorescent material directly to a resin system have been unsuccessful, mainly due to the settling away of high density phosphorescent material from the surface of the final article; and 2) the overall relative opacity of the resin mixtures due to shielding by fillers, which prevents the phosphorescent materials from being charged which, in turn, prevents the glow from being visible.
The method of coating the fabric with a phosphorescent utilized by Vockel, Jr. et al. has still left a need for polyester thermoset resin systems in which the phosphorescent pigments do not settle during storage and use and a need for polyester resin systems with suitable transparency and/or translucency characteristics for better utilization of phosphorescent particles. Such thermosetting luminescent resins would be extremely useful as thermosetting resins have properties making them suitable for screen printing. There is a need for improved luminescent thermosetting resins, methods and products in screen printing applications where reinforcing fabrics are not utilized.
Thermosetting resins encompass a wide range of materials including, for example, polyesters, vinyl esters and epoxies. Unsaturated polyesters are well known in the art and have been extensively studied and described.
Gel coats were introduced when thermosetting polyester resins were first being introduced for use with fiberglass or other fiber reinforcements. It was noticed in molded parts that the surfaces showed a distinct three-dimensional fiber pattern caused by shrinking of the resin away from the glass fibers during curing. Since these early parts were utilized almost exclusively for aircraft, this could not be tolerated for aerodynamic and aesthetic reasons. A remedy was soon developed in the use of gel coats, which today are utilized on the surface of thermosetting polyester plastics to produce a decorative, protective, glossy surface Which requires little or no subsequent finishing. Gel coats are used extensively for such items as shower stalls and bath tubs, outer surfaces of boats, campers, automotive bodies, swimming pools and a host of other parts and surfaces where a smooth, hard, tough and colored surface is a necessity.
As has been mentioned, one problem with utilizing phosphorescent pigments (which may have a specific gravity of 3.5 to 4 or more) in polymer resins is the tendency of the phosphorescent pigment to settle during blending operations and storage, particularly the larger size particles. Usually known luminescent polymers must be blended and utilized immediately, often with air equipment to keep the phosphorescent particles in suspension. This is also true of thermosetting laminating and casting resins, where typically the phosphorescent particle falls out of suspension and cannot be sprayed or conveniently worked. Thus, there is a particular need for polyester thermoset methods and products which keep the phosphorescent particles in suspension not only during blending and application, but also during storage over the useful life of the luminescent polymer.
An additional problem arises when attempting to utilize a phosphorescent pigment with polyester gel coats. If a phosphorescent particle such as an activated zinc sulfide is added to a gel coat, typically the phosphorescent particles separate out and the mixture overcongeals (similar to adding too much flour to water). An unmet need, therefore, remains apparent for phosphorescent polyester gel coats as well as moldable resins, which has not been provided by the prior art.
Electroluminescent devices were evidently first proposed by Destrau in 1947. Such a lamp may comprise a sheet of glass or plastic with a conductive layer which acts as a first electrode, an electroluminescent layer comprising phosphor in a binder and a conductive sheet on the other side of the electroluminescent layer which serves as a second electrode. When a voltage is applied across the two electrodes, the phosphor will emit light.
For example, U.S. Pat. No. 4,916,360 (1990) to Mikami et al. discloses a thin film electroluminescent device that comprises an electroluminescent film made with zinc sulfide serving as its host material and doped with a rare earth element to provide luminescent centers, insulating layers sandwiching the film and a pair of electrodes on the outer surface of the insulating layers. The EL film preferably has a ratio of sulfur to zinc atoms (S/Zn) of about 1.02≦S/Zn≦1.13, adapted to achieve an increased excitation efficiency at the luminescent centers to exhibit improved luminescent brightness. Rare earth elements having atomic number 59 to 69 (Pr to Tm) are suitable for doping, among which terbium, samarium, europium and praseodymium are desirable and selected in accordance with the desired luminescence color. The film is doped with the rare earth elements in an amount of 0.5 to 3 atom %. U.S. Pat. No. 3,740,616 (1973) to Suzuki et al. discloses electrically luminescent display devices which can be controlled to display characters or patterns. The display devices employ plural-gapped electrodes and multiple layers including an electrically luminescent layer. The electrically luminescent layers disclosed include a composition of zinc sulfide powder activated with copper and aluminum and a plastic binder such as urea resin, zinc sulfide powder activated with copper or manganese in thin film form, cadmium sulfide or silicon carbide luminescent materials and ZnCdS:Ag luminescent material. An insulating layer such as polyester film or barium titanate and a plastic binder which is white in color may be utilized and reflects the luminescence emitted from the electrically luminescent layer, thus intensifying the light output. U.S. Pat. No. 4,665,342 (1987) to Topp et al. discloses polymer luminescent displays formed of a matrix of individual light emitting elements adapted for excitation from a voltage supply. The electroluminescent displays can be manufactured using printed circuit and screen printing techniques. The matrix is formed on a substrate and each of the light-emitting elements comprises a first electrical conductor overlying the substrate, a dielectric with relatively high dielectric constant overlying the first electrical conductor, a light-emitting phosphor embedded in a polymer binder overlying the dielectric, and a second light transmissive electrical conductor such as indium oxide or indium oxide/silver polymer overlying the phosphor and defining a window for enabling viewing of the electrically excited phosphor. A polymer dielectric with a relatively low dielectric constant separates each of the individual light-emitting elements from each other and alleviates cross-talk between the individual light-emitting elements. These examples point out a continuing need for improved phosphorescent polymers for electroluminescent applications.
In summary, there remain various needs and unsolved problems which must be overcome before thermoset polyester resins can be most effectively utilized with the various phosphorescent particles. An effective thermoset resin must be water-resistant, protect UV sensitive phosphorescent pigments and provide a means for keeping heavy phosphorescent particles in suspension during storage and use. Such thermoset resins should have acceptable optical properties for use with phosphorescent pigments. An ideal thermoset phosphorescent polyester resin could be used or easily modified for use in screen printing and would have excellent photoluminescent, thermoluminescent and electroluminescent properties.